I have a form that displays large lists of selectable records using ModelMultipleChoiceFields. These fields need to show a predetermined list of records, obtained through some database queries performed within the Form's class init block.
The QuerySet required is constructed by reaching through a few ForeignKey relationships to retrieve the required records and then using the .union() method to assemble the QuerySet.
e.g.:

# Retrieve original record:
base = BaseModel.objects.filter(source_name=source_name).first()
# Get related record via ForeignKey relationship:
related_record = base.related_record
# related_record.other_related_things has ManyToMany relationship to another Model (one or more records).
# So get them:
other_related_things = related_record.other_related_things.all()
# Loop through these:
union_queryset = None
for other_related_thing in other_related_things:
# Each other_related_thing has a set of SelectableRecords associated with it, via a ForeignKey relationship from the latter model to the former one. This gets that QuerySet:
other_related_thing_queryset = other_related_thing.selectablerecords_set.all()
# Build union_queryset:
if union_queryset is None:
union_queryset = other_related_thing_queryset
else:
union_queryset = union_queryset_object.union(other_related_thing_queryset)
# The resulting union_queryset is a unified queryset containing all of the required fields.
# If only one other_related_thing exists, the result of evaluating the union_queryset QuerySet ought to be the
# same as the result of evaluating SelectableRecords.objects.all()

However, when I pass the union_queryset to my ModelMultipleChoiceFields, validating the form returns all of the records contained in union_queryset instead of the records selected on the form. The form displays correctly.

Do take note that the documentation for the .union() method (​here) indicates there are some limits to the QuerySet returned, and I assume my problem may have to do with this. Still, it would be welcome if this were explained somewhere (should this behaviour be normal), as it is rather unexpected.

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After some further testing it appears the unexpected behaviour comes from the fact that UNION querysets cannot be filtered, which the ModelMultipleChoiceField apparently does out of sight. Still, I believe that the observed behaviour is undesired, and the field should either accept the union queryset and work properly, or raise an exception if a union queryset is passed.

should we raise an exception for all filter() attempts? or just for unsupported one?
if we should we raise an exception just for unsupported, is there any way to find them?

Unsupported ones, I'd think. For my original problem, a simple check would be to do a count on the number of selections made in the form and comparing that to the number of records returned - normally they should match.